Cytomegalovirus (CMV) disease is a relatively frequent and often serious complication in immuno-compromised CMV-infected patients. In the last few years it has become apparent that, in order to differentiate between subclinical viral shedding and large scale viral replication occurring during the prodrome before the onset of active disease, it is necessary to utilize sequential monitoring with a quantitative assay. Several studies have shown that CMV quantitative polymerase chain reaction (PCR) assays are more sensitive than buffy coat CMV antigen detection assays. This extra sensitivity can in some cases give an additional week of warning before the onset of CMV disease in a patient Instituting anti-viral therapy at an earlier time point in the prodromal stage may decrease a patient's risk of developing active CMV disease. We are developing a competitive quantitative PCR assay to detect CMV in buffy coat cells. A standard amount of mimic of the DNA target sequence is included in the reaction mixture of each PCR tube so that variations in tube-to-tube PCR efficiency are detected and accounted for in the calculations of viral copy number made from the measured signal strength. The assay has the capability of detecting as few as 3 to 5 viral genome equivalents in an amplification reaction. Preliminary comparisons of the quantitative PCR protocol with p65 antigenemia determinations in a series of patient samples demonstrates that the PCR assay has greater sensitivity and permits an earlier detection of the CMV prodrome.